This invention relates to elements mounted in a cartridge for separating pure water from brine. More particularly, it relates to elements having a reverse osmosis membrane disposed entirely around the surface of a backing member which is porous throughout its length, width, and depth but which has sufficient structural strength to support the membrane. In addition, the invention relates to a method of manufacturing elements which comprises sequentially feeding a continuous strip of backing material through a heater and cold iron to provide a curved cross-section to the material, passing the material through an extrusion nozzle to extrude a reverse osmosis membrane on the backing material, passing the coated material sequentially into a chill bath and a heat treatment bath, cutting the material into discrete units and soaking the units in glycerine.
The use of semi-permeable membranes to separate liquids from salts dissolved therein is a well-known, commercially practiced technique. In recent years, the use of units in the home to purify water for drinking, ice making, and the like has become quite prevalent. Units used in the home are generally relatively small, since they must be placed underneath a sink or in other confined spaces, yet must maintain sufficiently high throughput to provide an adequate supply of purified water. Accordingly, it is necessary to design a unit which reasonably maximizes the membrane surface area per unit of volume of the cartridge, since the volume of purified water produced by a membrane element is generally directly proportional to the surface area of the membrane in contact with the brine.
It is common for membrane elements to be mounted in parallel fashion within a cylindrical cartridge, with purified water being collected in a header or manifold at one portion of the container, and with brine passing through the container around exterior surfaces of the membranes. Cheng, U.S. Pat. No. 3,612,282 and Manjikian, U.S. Pat. No. 3,544,358, are examples of such systems. Many other mounting configurations, such has spiral mounts of the type shown in Westmoreland, U.S. Pat. No. 3,367,504, Bray, U.S. Pat. No. 3,417,870, and Bairinja, U.S. Pat. No. 4,299,702, are also well-known. Difficulties have been encountered with spiralwrapped membranes in that separating members between helical wraps impede the flow of brine and also plug the membrane with salt. As a consequence, feed water must be filtered prior to feeding into a spiral-wrapped membrane. In addition, these membranes are relatively costly to manufacture and assemble. Also, in the past, in order to provide sufficiently dilute brine to the membrane surface, it has been necessary to have a continuous flushing of brine through the cartridge, resulting in a wasting of water. Continuous flushing of reverse osmosis units to drain has been criticized by many water districts and is expected to be precluded by law in a number of areas.
A substantial pressure drop is required across the reverse osmosis membrane, thus providing the driving force for separating the pure water from dissolved salts. The membrane itself, however, is quite delicate and is required to be supported to withstand the pressure. The support must be sufficiently strong to resist deformation by the external pressure, but must also be permeable to permit the flow of pure water passing through the membrane to be conducted into a collection chamber for usage. Resistance to passage of purified water through the porous membrane support will of course hinder the flow of water through the membrane, thus decreasing throughput of the element. Accordingly, much research has been done to discover a optimum element design. Many commercial units designed for purification of residential water have membranes cast over tubular backings, with water being collected inside the tube and being passed to a collection chamber.
In the present invention, a reverse osmosis element is made from a backing consisting of a stiff piece of porous plastic on which a reverse osmosis membrane has been cast. The element is porous laterally, longitudinally, as well as transversely; i.e., the backing is porous in three dimensions and water may travel in any direction through the backing once it has passed through the membrane. The membrane is cast around the entire periphery of the backing, and when it is mounted in a cartridge, the water passes through the membrane and into the backing, and travels along the backing toward one end which is mounted in a manifold. The elements may be arranged in a plurality of different geometrical configurations within the cartridge; since the elements are flat and have a slightly curved cross-section, they are mounted in configurations similar to the vanes in a turbine, thus providing relatively turbulent flow around both sides of the element and providing good scouring of the blade surfaces. Because the elements themselves are relatively rigid, they may be mounted in a cartridge without spacers which would touch the elements and possibly provide breaks therein when salts accumulate.
Elements of the invention are manufactured by feeding a continuous band of thermoplastic material and passing it through a heater to soften the material and then through an iron which sets the material to a desired curved cross-section. The band passes from the iron into an extruding nozzle within which a reverse osmosis membrane is extruded onto the surface of the backing, completely coating all peripheral areas. The coated backing is then passed through a chill bath to set the membrane, and a heating bath to heat treat the solid membrane. The band may then be cut into discrete elements and subsequently treated to remove water molecules from the element, e.g., with a glycerine bath. The elements are then mounted in parallel fashion in a cartridge in any of a variety of manners, some of which are described herein.
Accordingly, it is an object of the present invention to provide a reverse osmosis water purification system which is inexpensive to manufacture, and which has a relatively low failure rate of membrane in service. It is a further object of the invention to provide a reverse osmosis element consisting of an elongate flat member having a slightly curved cross-section consisting of porous backing entirely coated with a reverse osmosis membrane. It is yet a further object of the invention to provide a reverse osmosis cartridge having a plurality of flat curved elements mounted generally parallel therein, with one end of the elements extending through a header and into a collection chamber wherein purified water passing through the membrane backing is collected. It is yet a further object of the invention to provide a continuous method of manufacturing the discrete membrane elements having a backing consisting of a flexible plastic band. These and other objects are accomplished by the invention, several preferred embodiments of which are described herein.